The present invention relates in general to printing machines of the type which utilize a screen that has been coated with an emulsion and has been selectively exposed to present a stencil or pattern on the screen through which ink may flow for application to a substrate. More particularly, this invention relates to a method and apparatus for aligning the pattern in relation to the screen and then the screen in relation to the substrate.
Screen-type printing machines are widely used for the application of various types of inks to t-shirts, sweatshirts, baseball hats and other types of substrates to form patterns such as lettering and logos. The screens used in these types of machines are prepared by coating a photosensitive emulsion onto the mesh portion of the screens and allowing the emulsion to dry. The artwork pattern is then applied to the screen and the screen is exposed to a light spectrum which causes curing and hardening of that portion of the emulsion that is not covered by the opaque artwork pattern. The artwork is removed from the screen and the screen is washed to remove the uncured emulsion from the area of the screen which was shielded from the developing light. A stencil is thereby formed in the screen and after drying, the screen can be clamped in the printing machine and used to apply ink to the underlying substrate. Because the cured emulsion is impermeable to the ink, the ink under pressure flows through the screen only in those areas where the emulsion did not cure due to the presence of the overlying artwork.
In order to ensure that the pattern is applied to the substrate at the desired location, it is important that the screen be "registered" or accurately positioned so that the pattern formed in the screen is aligned on the substrate. This is particularly important when multi-colored patterns are being applied which require the use of multiple screens. In multi-colored processes, a portion of the desired pattern is applied to one screen while one or more portions of the pattern are applied to other screens. The screens are then clamped to rotatable arms which allow the screens to be successively positioned over the substrate to apply portions of the composite pattern. If even one of the screens are slightly misaligned, a poor quality composite pattern will result.
In many conventional processes, alignment of these screens on the printing machine is a time consuming process as the machine operator must visually align and clamp each screen. Typically, the operator will clamp the first screen and apply ink to a test substrate. Successive screens are then loosely clamped and registered by the operator visually observing their alignment in relation to the pattern applied from the previous screen. Each screen is then repositioned as required to bring the pattern into alignment. The screens are then firmly clamped and rechecked to ensure that the tightening of the clamps has not caused misalignment of the screens. As can be readily appreciated, this process requires considerable time and greatly reduces the productivity of the printing machines, particularly when only small production runs of a particular pattern are being produced.
Various methods have been utilized in an attempt to more accurately and more quickly align the screens both when the artwork is being applied to the screen and when the screen is being readied for clamping in the press. One such method utilizes a L-shaped bracket provided on the light box where the artwork pattern is applied to the screen. The screen is simply pressed into the corner formed by the bracket to quickly position the screen in relation to an underlying grid on the light box. While this method allows the screen to be quickly positioned on the light box, it does not positively hold the screen in place. The screen may therefor shift out of alignment as the operator releases the screen to position the artwork acetate onto the screen. Moreover, significant delays are still occasioned in aligning the stenciled screen on the printing machine after it has been prepared.
Another conventional method for aligning or registering screens utilizes brackets clamped onto the frame of the screens and fixed registration pins that are positioned on the light box and the printing boards. The brackets are positioned on opposite ends or sides of the screen and include a hole and slot arrangement for receiving the fixed registration pins. The cooperating brackets and pins thus ensure that the screens are properly aligned both during the application of the artwork to the screen and during the positioning of the screen in the printing machine. This method, however, is best suited for use with screens having one standard size because of the time required to alter the pin placement for screens of differing sizes. In addition, further delays are occasioned by the mounting and removing of the brackets from the screens.
A still further method used for the registration of screens employs a complex apparatus that utilizes optical sensors to read the position of alignment marks provided on the screen. Registration pins engage the screen and reposition it in response to the readings taken by the optical sensors. While this type of device can accurately position the screen during placement of the artwork acetate and in preparation for printing, it may require as long as 30 seconds or more to register the screen. Moreover, this type of machine is extremely expensive and many small businesses cannot afford to purchase this type of a registration system.